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Math Help - Fun little problem

  1. #1
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    Fun little problem

    Write the numbers 1 to 2n, where n is odd, on a white board. You are allowed to perform only the following operation: select 2 numbers, then subtract them and take the absolute value. Is it possible to end up with an odd last number? What about the case where n is even?
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  2. #2
    MHF Contributor Unknown008's Avatar
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    I'm not sure I understand...

    I choose n = 5. So, I write 1 up to 10.

    1, 2, 3, 4, 5, 6, 7, 8, 9, 10

    I take any 2: 8 and 9

    The absolute difference is 1... which is odd
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    Yeah, it's odd. Sorry, my statement of the problem may have been a little bit misleading. My question is: is it ever possible to end with an odd number after eliminating all but 1 of the numbers in this fashion. If it is, is the last number allways going to be odd? Could it possibly be even?
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    Quote Originally Posted by Chris11 View Post
    Write the numbers 1 to 2n, where n is odd, on a white board. You are allowed to perform only the following operation: select 2 numbers, then subtract them and take the absolute value. Is it possible to end up with an odd last number? What about the case where n is even?
    Well, if n=1, then you have 1,2 on that white board of yours.
    Select 'em, subtract: you get 1, so you end with an odd number........well?
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  5. #5
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    Prove that this happens for ALL n. I'm not asking for help. I just posted this because I thought someone would have fun with it--I did
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    Quote Originally Posted by Chris11 View Post
    Prove that this happens for ALL n. I'm not asking for help. I just posted this because I thought someone would have fun with it--I did
    BUT your question is (your initial post):
    "Is it possible to end up with an odd last number?"
    I showed it is possible.
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    Fine. Be a pedant.
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  8. #8
    MHF Contributor Unknown008's Avatar
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    I too don't quite understand the problem...
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    Quote Originally Posted by Chris11 View Post
    Fine. Be a pedant.
    Ahhh thanks; compliments will get you everywhere!

    OK; let's have a look at your problem as you posted it...
    n=1: 1,2
    n=3: 1,2,3,4,5,6
    n=5: 1,2,3,4,5,6,7,8,9,10
    ...and so on

    Here's what the puzzle is as you stated:
    > Write the numbers 1 to 2n, where n is odd, on a white board. You are allowed to perform only the following operation:
    > select 2 numbers, then subtract them and take the absolute value. Is it possible to end up with an odd last number?

    n=1: I select 1,2 ; diff = 1
    n=2: I select 1,2 then 3,4 then 5,6 ; diff's always 1
    n=3: I select 1,2 then 3,4 then 5,6 then 7,8 then 9,10 ; diff's = always 1
    So I end up with an odd number in ALL cases.

    I don't know what else to tell you.
    Last edited by Wilmer; November 1st 2010 at 01:08 PM.
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    Okay. I apologize. I should have reread my initial post after your initial reply. I forgot to add a critical part of the problem to my post. After you perform the operation, you erase the 2 numbers that you used with the result of the operation. You continue this process until you have one remaining number. Will the process always terminate with an odd number? An even number? Is it possible for it to terminate with an odd number? An even number?
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  11. #11
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    Quote Originally Posted by Chris11 View Post
    After you perform the operation, you erase the 2 numbers that you used with the result of the operation.
    Sorry, BUT whadda heck does that mean? Erase the 2 numbers, then what?
    Insert the number that you got from the operation?
    1,2,3,4,5,6
    I select 1,4; erase 1,4 and put in a 3: 2,3,3,5,6 are left?
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  12. #12
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    Yep. That's what you do
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  13. #13
    MHF Contributor Unknown008's Avatar
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    Well, now that it's clearer, I'm always getting odd numbers, except when I get 0.
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  14. #14
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    Ya UnKnown; stuff like:
    1,2,3,4,5,6
    select 6,3: 1,2,3,4,5
    select 5,2: 1,3,3,4
    select 1,4: 3,3,3
    select 3,3: 0,3 ??????

    Wonder if he means:
    1: numbers from 1 to 2n where n is an odd integer are written down
    2: you select any 2 numbers you wish and let k = absolute difference of these 2 numbers
    3: you then remove the 2 numbers from the list, and insert k
    4: you continue doing this until the last number in the list is the last inserted k
    5: can you devise a strategy where k will always be odd?
    (would need a rule applying to zero results)
    Last edited by Wilmer; November 2nd 2010 at 06:15 PM.
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  15. #15
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    Considering only the even/odd parity of the numbers (i.e. numbers modulo 2), it does not matter whether we are adding or subtraction, or taking absolute value. The last number is some arithmetic addition of all 2n numbers where some are positive and some negative.

    The ending parity will be the same as the parity of the sum of numbers 1 to 2n.
    Which is (2n + 1)n, and this is always odd for odd n.
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